Laser apparatus having a layer of silver and a layer of silicon monoxide



Jah 16, 1968 F. z. KElsTx-:R ETAL 3,363,998

LASER APPARATUS HAVING A LAYER OF SILVER AND A LAYER 0F SILICON MONOXIDEFiled March 16, 1964 Era-Z.

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f7/r 5. v gg@ United States Patent O ABSTRACT @F THE DISClLOilURlEl Alaser pumping cavity structure having a highly rellective cavity surfaceof a metal such as aluminum, copper, nickel, etc., coated with silver,in turn coated with a protective nonabsorbing dielectric film.Alternatively, chromium may be deposited on the cavity surface ahead ofthe silver or, as a further alternative, chromium and a co-depositedlayer of chromium and silver may be deposited on the cavity surfaceahead of the silver- This invention relates generally to highlyreflective laser pumping cavities and to methods of making suchcavities.

A laser requires a highly reflective pumping cavity to efficientlycouple the output of the flash tube or other source of radiant energyfrom the source to the laser element such as a ruby crystal.

Laser pumping cavities have been fabricated of a variety of materialswhich are suitable to the laser pumping cavity environment, which arecapable of being shaped to provide an internal cavity having a desiredconfiguration and which are additionally suitable to be provided with,or to have formed thereon, a cavity surface having the desired degree ofreflectivity to permit and support laser action. Probably one of themore common materials presently employed in the fabrication of lasercells is aluminum. Aluminum is easily Worked and may be shaped andpolished to a geometrically accurate, highly lustrous surface finishhaving high reflectivity, to function as a reflector or to function as abase for receiving a selected reflecting material such as silver, or toreceive still another base material such as chromium on which a materialhaving high reflectivity is to be disposed. While these and othermaterials provide desirable reflecting surfaces they tend to be unstablein the laser environment, being degraded, for instance, by exposure tothe high intensity radiation of the flash tube, such as the Xenon flashtube used for laser pumping.

One object of this invention` is to provide a laser cell having animproved cavity reflecting surface.

Further to the preceding object, it is an object hereof to provide animproved reflector cavity protective surface in a laser cell 'which haslow absorption, which tends to become less absorbing in the presence ofultraviolet radiation and which is relatively inert in laser cavityenvironments.

A specific object of this invention is to provide a laser cell havng ametal-dielectric reflector cavity comprised of a metal having a highindex of reflectivity and at least one layer of a low absorptivitydielectric material.

A further object of this invention is to provide a method forfabricating a laser pumping cavity having an im proved reflectingcavity.

More particularly with respect to the preceding object it is an objecthereof to provide an improved method for vacuum depositing materialsover a surface or surfaces defining a laser cavity.

The aforesaid and other objects and advantages are achieved, accordingto one aspect of this invention, in a laser pumping cavity having acavity provided with a reflecting surface. The pumping cavity may beformed of aluminum, brass, glass or any other material which can bemachined or formed to the desired geometrical shape and which willaccept a high degree of polish or smoothness. Silver is deposited eitheron the cavity surface or on some other material on the cavity surface towhich the silver will adhere by vacuum deposition to provide ageometrically true and highly reflective cavity surface.

Environmental degradation of the silver surface is minimized orsubstantially obviated in the disposition of a selected one of lowabsorption, transparent, dielectric material over the reflecting silversurface of the cavity to protect the surface from environmentaldegradation. lny asmuch as the-cavity presents a silver front surfacefunctioning as a reflector, such dielectric materials must exhibit aminimum of absorption of light energy but yet function to protect thereflecting surface from any form of degradation detrimental tooperation. In this respect metal-dielectric layers comprisingcombinations of silver and silicon monoxide (Ag-l-SiO) have been foundto produce protected cavity reflectors having statisfactory levels ofreflectivity and environmental durability. In general, it has been foundthat the addition of a non-absorbing dielectric lm t0 a metallic silverreflecting surface maintains reflectivity and at the same time providesa durableV protective coating in laser cavity environments and which,further, in some cases in the presence of ultraviolet flash tuberadiation becomes less absorbing of light energy, providing improvementsin cavity reflectivity after repeated exposure to flash tube radiation.

According to another aspect of this invention, the fabrication of laserpumping cavities, the reflecting surfaces of which are coated withprotective low absorptivity dielectric films, or having cavities coatedwith materials having reflective surfaces which are protected withdielectric materials, is achievable by means of a method employing thedeposition of materials by the process of evaporation. Such processesare normally conducted in a vacuum in which the surface to be coated iseX- posed to a specific material at an evaporation source. Suchprocesses normally require substantially uniform access of theevaporated material to the surface to be coated. inasmuch as the cavityof the laser pumping cavity presents a continuous interior surface, thatis, usually a suitably shaped surface of revolution or other closedsurface, the requirement for substantially uniform access to the surfaceto be coated without overheating the substrate presents a problem. Thishas been solved according to the present invention by providing segmentsto form the laser pumping cavity, either by sectioning after forming asa single-piece pumping cavity, or by fabrication in several segmentsinitially. The cavity faces may now be exposed to the evaporation sourcefor coating by vacuum deposition.

Improvements in reflectivity of reflecting surfaces is achieved inprovisions in the process of depositing of selected materials forcontrolling the rate at which deposition takes place and for controllingthe thickness. In general, the higher the rate of deposition of theevaporated reflecting material, the higher the reilectance will be,primarily due to the fact that oxidation is minimized.

Other objects and advantages will lbecome apparent from a study of thefollowing specification when considered in conjunction with theaccompanying drawings in which:

FlG. l illustrates a typical laser pumping cavity;

FIG. 2 illustrates a laser pumping cavity comprising four segments;

FIG. 3 schematically illustrates a typical vacuum deposition systemillustrating one aspect of this invention; and

FIGS. 4 and 5 are fragmentary cross-sectional views of a` laser pumpingcavity showing the construction of the improved multi-layer cavityreflectors.

FIG. l illustrates a typical laser pumping cavity 1 which is ofgenerally cylindrical configuration and which is provided with a cavity2, also of cylindrical configuration, extending axially therethrough andopening through the end faces of the pumping cavity. Such a pumpingcavity may be formed of any suitable material compatible with the laserpumping cavity environment and requirements. For the purposes of thisdiscussion aluminum will be referred to as the material from which thelaser pumping cavity is formed. This is one of the more frequentlyemployed materials for this purpose. To this end the pumping cavity 1may be a solid bar of aluminum having a cavity opening therethrough ofapproximately 1% inches in diameter. The cavity surface is accuratelymachined and highly polished to provide a highly finished, geometricallyaccurate reflecting surface. In the past this polished cavity surfacehas been employed as the reflector surface for the laser pumping.Although aluminum has relatively high reflectivity and, if carefullyfinished, can be used for this purpose, the reflectivity diminishesrapidly with use in the laser environment. As reflectivity diminishesthe efficiency of the laser drops off markedly, increasing the thresholdfor laser action and increasing the amount of pumping energy required toachieve laser action.

Efforts to overcome this type of problem have resulted in experimentingwith different types of reflecting surfaces in the laser cavities. Inthis respect silver has been found to be a good reflector and to have alonger life expectancy in the laser pumping cavity environment thanaluminum, for instance, but here again deterioration after limited usein the laser cavity environment results in low-- ering efficiency.Continuing experiment has resulted in the use of low absorptiondielectric materials over the reflecting metal surfaces. Theseexperiments have covered multi-dielectric layers as well as singledielectric layers. Some of the multi-dielectric layer arrangements haveindicated promise showing reflectivities of in the range of 96% to 97%for wavelengths of 4000 to 6000 A. One such multilayer reflectorinvolved highly polished aluminum as the reflecting material andone-quarter wavelength thickness deposits of silicon monoxide SiO andtitanium dioxide TiO2. Here, again, the life expectancy was too short,even though initial high rellectivities were indicated. Continuedexperiments resulted in a combination of silver and silicon monoxideAg-l-SiO as a multilayer reflector which produced high reflectivity andwhich exhibited stability in the laser cavity environment beyond anyother of the multilayer combinations that had been previously developed.

With respect to the silver and silicon monoxide reflector, reference maybe made to FIGS. 4 and 5. In FIG. 4 there is illustrated a fragmentarysection of the pumping cavity, generally designated 1, which may be someother substrate than aluminum, including nonmetallic materials. Thisrepresents a section of the laser cell.

If the material from which the cavity is formed cannot be polished tothe required degree of smoothness, a layer of some metal such as nickelor copper may be deposited chemically or electro-chemically. These andother metals having similar properties may be polished to a high surfacefinish, thereby forming an acceptable surface for the deposition of thesucceeding layer.

Next follow a layer 4 of chromium, a co-deposited layer of chromium andsilver, a layer 5 of silver and a layer 6 of silicon monoxide. The layer3 provides a satisfactory base material over the substrate materialpermitting forming of a geometrically accurate and highly reflectivesurface. Chromium or nickel-chromium is next deposited over the layer 3to provide a layer to which the subsequent silver layer will adhere. Thesilver layer is next applied over the chromium and is finished to a CIIhigh lustre, again of geometrically accurate configuration, after whichsilicon monoxide is deposited to complete the multilayer reflector andto provide a protective coating for the silver surface.

In FIG. 5 the substrate material of the laser pumping cavity is assumedto be aluminum. After polishing of the cavity surface the sequence ofdeposition of chromium, co-deposited chromium and silver, silver andsilicon monoxide follows as in connection with FIG. 4.

Although chromium has been indicated as a material desirably appliedover the substrate, it is to be understood that other materials, such ascommercially available nickel chromium alloys affording somewhat similarproperties, may be employed.

Laser pumping cavities utilizing the multilayer rellector constructiondescribed herein, and particularly ernploying the silver-siliconmonoxide reflector layer construction have been tested and have showndrastic decreases in the threshold value for laser action, together witha general increase in laser efficiency when compared to polishedaluminum surfaces and have shown a substantial decrease in the thresholdof laser action, together with an increase in laser efficiency whencompared to vacuum deposited aluminum surfaces overcoated with siliconmonoxide. Measurements of samples which have been repeatedly exposed alarge number of times to flash tube radiation during laser pumping haveshown no appreciable decrease in reflectivity as has been noted forunprotected silver reflectors, unprotected aluminum reflectors, aluminumreflectors protected with either silicon monoxide or titanium dioxide,or both, and others. Additionally, it has been observed that absorptionof high intensity ultraviolet from the flash tube radiation causes thesilicon monoxide coating to become less absorbing and thus yielding insome cases an increase in reflectivity after repeated exposure to flashtube radiation. In this respect it is believed that the decrease inabsorption may be the result of formation of silicon dioxide SiO2, S203,or other SiOX structure in the dielectric coating.

The techniques for fabricating laser pumping cavities of the type hereindescribed are based upon the vacuum deposition of thin films of selectedreflective and dielectric materials of the type described hereinabove,although the silver layer may be a layer of silver foil or may be alayer of electro-deposited silver. In general, the descriptivedisclosure which follows, directed to the method of making the improvedlaser pumping cavity, will be directed to procedures involving analuminum pumping cavity having a highly reflective geometricallyaccurate cavity, the surface of which forms a substrate for thedeposition of the named materials. In these discussions it will beappreciated that the function of the thin film or bulk layer of silveris to provide high reflectivity and the function of the silicon monoxidefilm is to protect the silver surface from dust and other contaminantsand to prevent oxide formation or other unwanted reactions, particularlyin the laser environment, which might decrease or degrade reflectivity.

Conventional practices in the preparation of thin films by the processof vacuum deposition involves the use of a suitable chambercommunicating with a vacuum pump capable of pumping the chamber down toa particularly desired low pressure. The chamber includes an evaporationsource which comprises the material which is to be evaporated to formthe thin film. Such a source is usually heated electrically in such away as to provide a controlled input of electrical energy to providerates of evaporation necessary to achieve desired rates of deposition ofthe material on the particular substrate or other surface to which it isto be applied. In vacuum deposition processes, it is necessary that theproducts of evaporation have substantially uniform access to the surfaceto be coated. In laser pumping cavities of the general size describedherein, having a cavity of about 13/8 inches in diameter this presents aproblem since while conceivably a source may be positioned in a positionsubstantially centrally and extending axially of a cavity, such as thecavity 2 in the laser pumping cavity, the proximity of the hot source tothe cavity walls results in undesirable heating of the substratematerial, interfering with satisfactory coating or deposition ofmaterials on the cavity surface.

The present invention overcome this problem by providing a laser pumpingcavity which is fabricated of segments. An arrangement of this type maybe formed of individual segments which are precisely fabricated or thepumping cavity may be made as a single piece, as illustrated in FIG. 1,and thereafter carefully sectioned to provide the cylindrical segments1a through 1d, as illustrated in FIG. 2. The cylindrical segmentscomprising the laser pumping cavity, as will be seen in FIG. 3, aresupported beneath a substrate holder 8 disposed within a casing 9forming part of a vacuum deposition system, generally designated 10.Such a casing 9 may be of stainless steel or glass. The substrates 1athrough 1a.' are suspended with their cavity faces directed downwardly,pointing in the direction of an evaporation source 11 on electrodes 12above a base plate 13 of the assembly. The interior volume communicatesthrough a conduit 14 with a suitable vacuum'pumping system, not shown,so that the interior volume may be lowered in pressure to that necessaryfor satisfactory evaporation and deposition of the source material.Immediately above the substrate support 8 is a heater, generallydesignated 15. This is preferably an electrical heater which may becarefully controlled to provide precise substrate temperatures toachieve optimum coating of the substrate during the vacuum evporationprocess. Facilities may be provided for monitoring both the evaporationrate 4by means of a rate monitor 16 and for montoring the thickness bymeans of a thickness monitor 17 which are coupled to suitableinstrumentalities, not shown, for the purpose of providing informationnecessary in adjusting the control of the evaporation source 11 and/orthe substrate heater 15. The vacuum evaporation system includesadditionally a shutter 18 which extends over the evaporation source 11and which may be pivoted about a xed pivot 19 at the upper end of thesupport 20 projecting upwardly from the base plate 13. When the shutter18 is removed the substrates are exposed to the evaporation source 11.Vacuum evaporation systems having rate and thickness monitoring controlsare described in co-pending applications Serial No. 249,044, now PatentNo. 3,297,944, issued I an. 10, 1967, entitled Rate Monitoring Systemfor Vacuum Evaporation Process," inventors Paul Nektaredes and R. Y.Scapple, led Ian. 2, 1963, and Ser. No. 227,- 228 (abandoned Mar. 14,1966) entitled Film Thickness Monitoring System, inventors R. B.Feuchtbaum and R. Y. Scapple, both applications being assigned to theassignee of this invention.

In evaporation processes the uniform application of the evaporatedmaterial as a coating over a substrate requires substantially uniformaccess of the evaporated material to the substrate surface. Preferably,the substrate surface will be approximately normal to a line from thesubstrate to the evaporation source. This idealized arrangement is notcompletely available in this situation in lview of the arcuateconfiguration of the cavity surfaces but is approximated to a sufficientdegree to permit adequate and satisfactory coating, particularly when itis realized that migration of the products of evaporation from thesource to the substrate surfaces involves some random motion o'f theproducts of evaporation.

In the process of fabrication of one specific laser pumping cavity,silver wire, chromium powder and silicon monoxide were employed as theevaporation materials.

Chromium powder which was about 99% plus pure and which is commerciallyavailable was placed in a tungsten boat from which it was evaporated byelectrical heating.

6 The tungsten boat is also a commercially available item usable in suchprocesses.

Silver wire which was about 99.9% pure and approximately 0.020 inches indiameter was evaporated from a double molybdenum boat which waselectrically heated. Such boats are also commercially available, as isthe sil-ver wire.

The silicon monoxide was a commercially available +10 mesh, vacuum bakedproduct. It was evaporated from a tantalum chimney source which is alsocommercially available.

The silver, chromium and silicon monoxide evaporation were done in aconventional vacuum coater with a stainless steel bell jar.

Thickness measurements of the thin films were made by multiple beaminterferometric techniques.

In practicing the process the four laser pumping cavity segments 1athrough 1d were mounted in the vacuum chamber beneath the substrateholder 8, as indicated, at a distance of about 12 inches above theevaporation source 11. A small aluminum disc 21 is also mounted beneaththe substrate holder 8 at one side of the segments, for example,adjacent the segment 1d to act as a control specimen for rellectivitymeasurements.

In the deposition of the chromium and siliver the substrates werepositioned about 10 inches from the evaporation source 11. In thisinstance two evaporation sources, one chromium and one siliver, wereemployed. Chromium was lirst evaporated on the substrate to a thicknessof about 400 A. At this time the silver was gradually phased into theevaporation stream by heating the silver evaporation source, thusyielding a chromium and silver codeposited layer. After about 30 secondsof deposition of the co-deposited layer the chromium evaporation wasstopped and the evaporation of the silver continued on the substrates toa silver layer thickness of about 750 A. at a rate of approximately 13A. per second. Chromium and silver are evaporated together for a periodof about 30 seconds while the rate of evaporation of the chromium isbeing reduced to zero and the rate of evaporation of the silver is beingincreased. Throughout this operation the vacuum was maintained at about1.5 104 torr and the substrate temperature was maintained at about 112C. In general, the faster the silver is deposited the better will be thedeposited silver layer since oxidation is minimized.

The silicon monoxide was next evaporated; the tentalum chimney sourcecontaining the silicon monoxide now constitutes the evaporation source,The laser pumping cavity segments were disposed at a distance of aboutl0 inches from this source. Silicon monoxide was evaporated so as toprovide a deposition rate of from 11 A. per second to 13 A. per secondto a thickness of about 450 A.i50 A. During this process the vacuum wasmaintained at 5x10-5 to 3x10*5 torr and the substrate temperature wasmaintained at about 70 C. In general, the rate of deposition of thesilicon oxide may be lower than for the reflecting materials. Also thepressure in which evaporation is taken may be higher since exposure tosome oxygen is not objectionable.

Although the specific procedure for fabricating the improved laserpumping cavity as outlined hereinabove is directed primarily to apumping cavity fabricated of aluminum and is directed to a specificmultilayer reflector involving chromium, it will be appreciated by thoseskilled in the art that the process may -be practiced without the use ofa chromium coat or layer. Again, with reference to FIG. 5, and withrespect to the process outlined hereinabove, when a pumping cavitycomprised of aluminum is employed the chromium may be deposited directlyon the substrate cavity surface or, alternatively, only a silver andsilicon monoxide reflectorI may be deposited on such an aluminumsubstrate. In the extreme, silicon monoxide alone may be deposited overthe cavity surfaces to provide protection for the aluminum surface.

These and other variations will be apparent to those skilled in the art.

lnasmuch as material thickness must be fairly acclurately controlled,and since the range of thickness of the several layers is generallybelow that which may be satisfactorily monitored by many availablemonitoring controls, a method was devised whereby the evaporationsources could be operated at particular evaporation rates for specicperiods of time to achieve thickness of the layers as required. To thisend, statistical data was accumulated on the various evaporation sourcesby operating cach source in its environment and depositing theevaporated material upon test pieces supported in the vacuum chamber. Bynoting the energy level of the input to the evaporation source, the timeof its operation and the thickness of the material deposited, it waspossible to select particular energy inputs to achieve evaporationsource operation providing desired rates of deposition. Thus, by timingthe evaporation operation layer thicknesses controlled to within i25 A.were obtainable.

What is claimed is:

1. A laser pumping cavity structure comprising: a body structureincluding a metal portion having a highly reflective metal surfacetherein defining a cavity; a layer of silver disposed on said surface;and a layer of silicon monoxide disposed on said layer of silver.

2. A laser pumping cavity structure comprising: a body structureincluding a metal portion having a layer of chromium providing a highlyreilective surface deiining a cavity; a layer of silver disposed on saidlayer of chromium; and a layer of silicon monoxide disposed on saidlayer of silver.

3. Apparatus as set forth in claim 2 in which said layer of chromium isabout 400 A. in thickness and the combined thickness of said layer ofchromium and layer of silver is about 1200 A.

4. Apparatus as set forth in claim 2 in which chromium and silver areinterrnixed in changing amounts in the region between the layer ofchromium and the layer of silver.

5. Apparatus as set forth in claim 2 in which the thickness of saidlayer of silicon monoxide is about 450 A.

References Cited UNITED STATES PATENTS 2,539,246 1/1951 Hensel 29--1973,274,024 9/1966 Hill 117-215 HYLAND BIZOT, Primary Examiner.

